Back to amplifiers with weird issue

[Colas LeGrippa]

Hi buddies,


I have this litlle se princeton amp copy ( not exactly a clone , with a few mods, but very close) that gives me headaches.
When I turn it on , it hums once warmed up. If I shut it off, I hear a small "tik" and when I turn it on again,  the hum has disapeared. Every f....ing time it does the same. Power
 transformer, audio transformer ? The 3 x 50uF caps are new, heater circuit good, 6.34 vac  with 5w 100 R resistors to power tube K.
Tried to supply ac voltage from a variac, same issue. Once the hum gone, as long as the amp is on,  it works perfectly.


Any idea ?


Thanks


Colas LeGrippa

[shooter]

se princeton

do da basics;
monitor the vdc at cathode R, stable?
monitor VAC at cathode R, anything > than millivolts?
clean pa socket, swap in a different pa tube, still humm'n?

[sluckey]

Is this based on a 5F2 or 5F2A? Try another 5Y3. 100Ω is not nearly enough for a 6V6 cathode. Fender used a 470Ω.

[Colas LeGrippa]

It s based on a 5F2A.


THE  100R's  I was talking about was the heater reference,  dc elevated on the power tube K, not the K bias resistor of 470 Ohms, sorry if that wasn't clear


Zero VAC  on the power tube K, DC voltage very stable on both plate and K


I swapped the 6V6 for an EL34 , biased at  21W, well under the max.


Anode to K voltage: 360V , K voltage: 28V


A/C high voltage center tap from power transformer connected to the ground of the reservoir cap. as well as the OT B+ wire and choke to screen, which is hooked up to another filter cap


Main mods:  5Y3 to SS
3 filter caps swapped for 50uF
6V6 replaced by EL34
switchable .68 uF on the second k of the 12AX7
CHOKE added right after the OT B+ supply

[DummyLoad]

Schematic? Pics? 

--Pete

[DummyLoad]

You have 50/100uF filter on the 5Y3? 100uF probably going to arc/sputter, 50uF may arc/sputter. Try sub in pair of 1N4007 or something like the Weber "Copper Cap" or the Tube Depot SSR Octal plug-in, see if issue follows.


--Pete

[Colas LeGrippa]

I mentioned I swapped the 5Y3 for a ss rectifier, so no worries about the 50uF first cap

[Colas LeGrippa]

The amp just works fine. As I said, it's humming when turned on after a while, even if the volume is off and nothing is plugged in.. The fact of turning it off and immediately on again gets rid of the noise completely, almost dead quiet at full volume.
I took my cell  phone away from it, I plugged it in the living room: same issue. No florescent tubes or strong magnetic fields in my house

[acheld]

Kinda sounds like a charge is building up somewhere, and then getting discharged when you turn it off . . .        Leaky cap??   

I feel yer pain on this one.

[Colas LeGrippa]

Yeah, kind of.....
Leaky cap ? I doubt it


Maybe a bypass cap on v1 cathode

[Rontone]

Kinda sounds like a charge is building up somewhere, and then getting discharged when you turn it off . . .

switchable .68 uF on the second k of the 12AX7

Could it be this cathode cap switch is intermittent? Or causing a problem across both bypass caps when charging?

[Colas LeGrippa]

Hi my friends,  I found out the first filter cap was defective after replacing it, though it s reading in uF was normal.
The amp is dead quiet now, tone back to normal, ready for recording with my new API rack and pres.
I must have been really rushy when I built this small amp. That specific hum problem gave me the "chance"  ,  opptunity should I say, to pin pont issues that I would never had detected otherwise.
Examples:  one 10K dropping resistor not soldered both sides,  220k resistor from signal to gnd at the power tube hooked up to the wrong side of the grid stopper, power tube bypass cap grounded to the preamp gnd, female chassis jack at speaker out defective as a matter of fact  the  OT wire broke loose while poking around with a chopstick, bad solder from the choke out to the power tube plate....
Anyway, I passed 3 days trying to make a great amp out of a shitty box. What would have occured with a much more complex amp with lots of IC ??? I just can t imagine...🤪
Thanks for tips and advices.


Colas LeGrippa
See ya in a couple of years 🤣🤣🤣